Apparatus and process for gas sensor control

ABSTRACT

A gas sensor control apparatus includes a heater regulating section to control the supply of electricity to a heater included in a gas sensor, an impedance sensing section to sense an impedance of the gas sensor cell, an impedance condition examining section to examine whether the impedance is greater than or equal to an abnormality threshold, a voltage condition examining section to examine whether a maximum effective voltage is applied to the heater, when the impedance is above the abnormality threshold, a duration measuring section to examine whether an application time duration of the maximum effective voltage becomes equal to or longer than a heater overheat preventing time, and a voltage decreasing section to decrease the heater voltage to such a lower effective voltage as to hold the temperature of the cell higher than or equal to 500° C. when the application time duration reaches the heater overheat preventing time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. application Ser. No. 12/612,979filed Nov. 5, 2009, which claims priority from Japanese PatentApplication No. 2008-285054 filed Nov. 6, 2008. The entire disclosuresof the prior applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and/or process forcontrolling a gas sensor, such as a gas sensor including at least onecell which includes a solid electrolyte member and a pair of electrodesformed on the solid electrolyte member. Specifically, the presentinvention relates to a technique for sensing the impedance of a cell ofthe gas sensor, and controlling the supply of electricity to a heater ofthe gas sensor in accordance with the sensed cell impedance.

A gas sensor, such as oxygen sensor and air fuel ratio sensor, is widelyused for an internal combustion engine for a motor vehicle or otherapplications, to improve the fuel consumption and control thecombustion. Furthermore, in conjunction with the environmental concernsand tightening of regulations for limiting exhaust emissions of motorvehicles, the demand for reducing the amount of nitrogen oxides (NOx) inthe exhaust gas mixture increases and drives the development of an NOxsensor capable of sensing the concentration of NOx directly. These gassensors use a gas sensing element including one or more cells eachincluding a solid electrolyte member of an oxygen ion conductive solidelectrolyte such as zirconia, and a pair of electrodes formed on thesolid electrolyte member. The gas sensing element produces an outputfrom which the concentration of a specified gas can be determined.

As the gas sensor, there is known a full range air fuel ratio sensor(also called UEGO sensor) for sensing the oxygen concentration of ameasurement gas with a construction which includes two cells (oxygenconcentration sensing cell and oxygen pump cell) disposed on both sidesof a measurement chamber to which a measurement gas is introducedthrough a diffusion resisting member. Moreover, there is known an NOxgas sensor including a cell for sensing the NOx concentration inaddition to the two cells (oxygen concentration sensing cell and oxygenpump cell).

The gas sensor is connected with a sensor drive circuit for supplyingelectricity to a cell of the gas sensor, and arranged to sense theconcentration of a specified gas in the measurement gas (gas mixture)from an output of the cell. Such a gas sensor includes a heater forheating the cell (the solid electrolyte member of the cell) to a highertemperature higher than or equal to an activation temperature so thatthe solid electrolyte member becomes oxygen-ion-conductive. The currentsupply to the heater is controlled to activate the cell quickly and tohold the cell active at temperatures equal to or higher than theactivation temperature stably.

A Japanese patent document (JP 10-48180A) shows a temperature controlsystem arranged to sense the impedance of a cell (element impedance orcell impedance) which varies in dependence on the temperature of thecell, and to control the temperature of the cell by controlling thecurrent supply to the heater. It is possible to control the temperatureof the cell by controlling the amount of electricity supplied to theheater so as to bring the impedance of the cell sensed periodically, toa desired target impedance. It is possible to sense the cell impedanceby inputting a impedance sensing signal (or current) into a monitoredcell whose impedance is to be sensed, and to determine the impedancefrom an output (response signal) of the monitored cell responsive to theimpedance sensing signal.

If a wiring line leading to the monitored cell is disconnected by sometrouble, the impedance of the monitored cell sensed by the controlsystem increases excessively or infinitely. As a result, the controlsystem misjudges that the cell (gas sensor) is cold, and continuessupplying the maximum effective voltage to the heater. The continuationof application of the maximum effective voltage might increase thetemperature of the gas sensor abnormally and damage the gas sensor.Therefore, a Japanese patent document UP 2000-121600) proposes adiagnostic system for detecting an abnormal condition such asdisconnection or short-circuit of gas sensing element and heater bymonitoring an unusual variation of the element impedance.

SUMMARY OF THE INVENTION

However, when the abnormality is detected, the above-mentioneddiagnostic system inhibits the supply of current to the heater, so thatthe gas sensor is cooled and foreign substance such as carbon may adhereto the surface of the sensor (the surface of the sensing element). Thematter adhering to the gas sensor might obstruct the function of the gassensing element when the normal condition is regained and the normalheater control mode is restarted. In view of such a problem, it is anobject of the present invention to provide technique, such as gas sensorcontrol apparatus and/or process, adequate for preventing improperoverheating of gas sensor due to trouble such as wiring disconnection,and enabling restoration of a normal gas sensor operation. It is anotherobject of the present invention to provide technique, such as gas sensorcontrol apparatus and/or process, adequate for preventing improperoverheating of gas sensor due to overheating of the heater when such acondition continues that an impedance of the gas sensor detected forwiring disconnection of the cell or the like, shows exceedingly highlevel, and preventing foreign substance such as carbon from adhering tothe surface of the sensor by cooling the gas sensor without stopping thesupply of current to the heater.

According to one aspect of the present invention, a gas sensor controlapparatus for controlling a gas sensor for sensing a concentration of aspecified gas, the gas sensor including at least one cell including asolid electrolyte member and a pair of electrodes formed on the solidelectrolyte member, and serving as a monitored cell, comprises: a heaterregulating section to control a supply of electricity to a heaterincluded in the gas sensor; an impedance sensing section to sense animpedance of the (monitored) cell of the gas sensor, from an output ofthe cell; an impedance condition examining section to examine whetherthe impedance sensed by the impedance sensing section is greater than orequal to a predetermined abnormality judging threshold; a voltagecondition examining section to examine whether a maximum effectivevoltage is applied to the heater, when the impedance is judged to begreater than or equal to the predetermined abnormality judgingthreshold; a duration measuring section to examine whether anapplication time duration of application of the maximum effectivevoltage to the heater becomes equal to or longer than a predeterminedheater overheat preventing time; and a voltage decreasing section tocommand the heater regulating section to apply, to the heater, a lowereffective voltage which is lower than the maximum effective voltage andwhich is so set as to hold the temperature of the cell higher than orequal to 500° C. when the application time duration becomes equal to orlonger than the predetermined heater overheat preventing time.

According to another aspect of the invention, a gas sensor controlprocess for controlling a gas sensor for sensing a concentration of aspecified gas, the gas sensor including a heater and at least one cellincluding a solid electrolyte member and a pair of electrodes formed onthe solid electrolyte member, and serving as a monitored cell,comprises: an impedance sensing step of sensing an impedance of the(monitored) cell of the gas sensor, from an output of the (monitored)cell; an impedance condition examining step of examining whether theimpedance sensed by the impedance sensing step is greater than or equalto a predetermined abnormality judging threshold; a voltage conditionexamining step of examining whether a maximum effective voltage isapplied to the heater, when the impedance is judged to be greater thanor equal to the predetermined abnormality judging threshold; a durationmeasuring step of examining whether an application time duration ofapplication of the maximum effective voltage to the heater becomes equalto or longer than a predetermined heater overheat preventing time; and avoltage decreasing step of applying a lower effective voltage which islower than the maximum effective voltage and which is so set as to holdthe temperature of the cell higher than or equal to 500° C., to theheater when the application time duration becomes equal to or longerthan the predetermined heater overheat preventing time.

According to still another aspect of the present invention, a gas sensorcontrol apparatus or process comprises part or all of the followingelements (sections or steps): a heater regulating element to supplyelectricity or electric power to a heater included in a gas sensor forsensing a concentration of a specified gas; an impedance sensing elementto sense an impedance of a (monitored) cell of the gas sensor; and acontrolling element to check a first condition which is satisfied whenthe impedance is greater than or equal to a predetermined abnormalityjudging threshold, to check a second condition which is satisfied whenthe electric power supplied to the heater is higher than or equal to apredetermined higher power level, to measure a continuance (or timeduration) of a third condition which is satisfied when the first andsecond conditions are both satisfied simultaneously, and to decrease theelectric power or the amount of electricity supplied to the heater, fromthe higher power level to a lower power level lower than the higherpower level when the continuance of the third condition becomes equal toor longer than a predetermined overheat preventing time. The lower powerlevel may be so set as to hold a temperature of the gas sensor at such ahigher temperature level preventing adherence of a substance containedin a gas to the gas sensor. The controlling element may comprise thefollowing subelements (subsections or substeps): a first subelement tocheck the first condition, a second subelement to check the secondcondition, a third subelement to measure the continuance of the thirdcondition, and a fourth subelement to decrease the electric power to thelower power level when the continuance of the third condition becomesequal to or longer than the predetermined overheat preventing time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a gas sensor control system accordingto an embodiment of the present invention, forming at least a part of aninternal combustion engine control system.

FIG. 2 is a circuit diagram schematically showing the construction of anelectronic control unit in the gas sensor control system of FIG. 1.

FIG. 3 is a graphic view for illustrating an overheat preventing processperformed by the gas sensor control system of FIG. 1.

FIG. 4 is a flowchart showing a heater control process performed by thegas sensor control system of FIG. 1.

FIG. 5 is a flowchart showing the overheat preventing process performedby the gas sensor control system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a control system 1 including a gas sensorcontrol unit (electronic control unit ECU) 5. The control system 1 ofthis example is constructed as an internal combustion engine controlsystem performing various control operations to control one or moreoperating conditions of an internal combustion engine (engine), andoperations for sensing a specified gas component (such as oxygen) in ameasurement gas (such as exhaust gas mixture) to be examined.

This control system 1 includes the electronic control unit 5 and a gassensor 8 attached to an exhaust pipe of the engine. Electronic controlunit 5 includes a sensor control circuit 2 for controlling the gassensor 8 (sensing element 10), an engine control device 9 (hereinafterreferred to as engine CPU 9), and a heater control circuit 60 forcontrolling a heater (or heating element) 43. The sensor control circuit2 includes a sensor drive circuit 52. The engine control device 9 isconnected with the heater control circuit 60, and configured to controlthe heater control circuit 60 so as to control the temperature ofsensing element 10 to an operation temperature (or activationtemperature). The engine control device 9 is connected with the sensorcontrol circuit 2 through a transmission cable 71, and configured tocontrol the sensor control circuit 2.

The gas sensor 8 includes the sensing element 10 for sensing the oxygenconcentration in a wide range in the measurement gas (exhaust gasmixture) which is the gas under measurement, and the heater or heatingelement 43 for holding the temperature of sensing element 10 at theoperation temperature. This gas sensor 8 functions as a full range airfuel ratio sensor. The sensing element 10 includes an oxygen pump cell14, a porous diffusion layer 18, an sensing cell 24, and a reinforceplate 30, as mentioned more in detail later.

The sensor control circuit 2 includes the sensor drive circuit 52connected electrically with gas sensor 8, and other circuits. The sensordrive circuit 52 controls gas sensor 8 by supplying electricity to gassensor 8 (oxygen pump cell 14 and sensing cell 24), and senses an outputsignal of oxygen pump cell 14 (gas sensor signal) and an elementimpedance (an impedance signal Vrpvs varying with the element impedanceof the oxygen concentration sensing cell 24, in this example). Sensorcontrol circuit 2 delivers the gas sensor signal and the elementimpedance signal to the engine control device 9. In this example, oxygenconcentration sensing cell 24 is a cell whose impedance is to be sensed,and hence oxygen concentration sensing cell 24 can serve as “(impedance)monitored cell”.

The sensor control circuit 2 can be realized in the form of ASIC(application specific integrated circuit), for example. The gas sensorsignal is a signal which varies in dependence on the oxygenconcentration of the measurement gas, and which is used for sensing theoxygen concentration. The element impedance (impedance signal)represents the electric resistance of gas sensor 8 which varies with thetemperature of gas sensor 8. The gas sensor 8 (heater 43) is controlledin accordance with the element impedance, as mentioned later more indetail.

The sensor control circuit 2 (sensor drive circuit 52) includes a Vs+terminal, a COM terminal and an Ip+ terminal which are connected,respectively, with a first connection terminal 15, a second connectionterminal 17 and a third connection terminal 19 electrically. The Vs+terminal of sensor control circuit 2 is connected, through firstconnection terminal 15 and a wiring line 61, to a later-mentioned secondsensing electrode 28 of sensing element 10. The COM terminal of sensorcontrol circuit 2 is connected, through second connection terminal 17and a wiring line 62, to a first sensing electrode 22 and a second pumpelectrode 16 of sensing element 10. The Ip+ terminal of sensor controlcircuit 2 is connected, through third connection terminal 19 and awiring line 63, to a first pump electrode 12 of sensing element 10.Thus, sensor drive circuit 52 is connected with gas sensor 8, andconfigured to receive the gas sensor signal and the impedance signal.

In the following explanation, no distinction is made between the Vs+terminal and first connection terminal 15, between the COM terminal andsecond connection terminal 17 and between the Ip+ terminal and thirdconnection terminal 19. In the following explanation, “Vs+ line” isused, according to the need, to denote the portion causing an excessiveincrease of the element impedance if a wiring trouble or abnormality(such as disconnection or breakage of wiring line) occurs in theportion. The Vs+ line includes at least the wiring lines 61 and 62extending from the sensing electrodes 28 and 22 of oxygen concentrationsensing cell 24 for sensing the element impedance, and the wiringsection of the sensor drive circuit 52 connected with the wiring lines61 and 62. A control section 55 can be constructed as an analogswitching circuit, for example.

The engine control device 9 can be composed of a microcomputer includinga central processing unit CPU, a memory section (RAM and ROM) forstoring information such as data and programs, and input and outputports for inputting and outputting signals from and to external deviceor devices. In engine control device 9, CPU performs various computeroperations according to a program stored in the memory section andcontrols the calculation, data transfer and the execution ofinstructions. Furthermore, engine control device 9 reflects a signalinputted to the input port on the contents of an input port register,and supplies the contents stored in an output port register, as asignal, to the output port.

In accordance with the gas sensor signal Vip outputted from sensorcontrol circuit 2, the engine control device 9 determines or judges theenergizing condition or the current supply condition (such as thecurrent direction and the current magnitude) of an Ip current flowingthrough oxygen pump cell 14, and calculates the oxygen concentrationfrom the energizing condition of the Ip current. By using the calculatedoxygen concentration, the engine control device 9 controls thecombustion and other operating conditions of the engine.

In accordance with the impedance signal Vrpvs outputted from sensorcontrol circuit 2, the engine control device 9 calculates the impedanceRpvs of oxygen concentration sensing cell 24, and performs a heatercurrent control process of outputting a heater current command forheater 43 in accordance with the calculated impedance Rpvs, to heatercontrol circuit 60. In one of possible interpretations, this heatercurrent command corresponds to “command”, and the engine control device9 corresponds to “impedance abnormality judging means”, “voltageapplication judging means”, “voltage applying state judging means”,“commanding means”, “activation judging means”, and “time settingmeans”, and can serve as at least part of “impedance condition examiningsection”, “voltage condition examining section”, “duration measuringsection”, “voltage decreasing section”, “activation judging section”,and “time setting section”. The heater control circuit 60 and enginecontrol circuit 9 correspond to “heater controlling means”, and canserve as at least part of “heater regulating section”. The sensorcontrol circuit 2 and engine control device 9 correspond to “impedancesensing means” and can serve as at least part of “impedance sensingsection”.

The gas sensor 8 has the following construction as shown in FIG. 1. Theoxygen pump cell 14 includes a solid electrolyte member 13 shaped like aplate, and first and second pump electrodes 12 and 16 formed,respectively, on front and rear surfaces of solid electrolyte member 13.The solid electrolyte member 13 is made of an oxygen ion conductivesolid electrolyte. In this example, solid electrolyte member 13 is madeof a partially stabilized zirconia (ZrO₂). First and second pumpelectrodes 12 and 16 are made of material containing platinum, as maincomponent. First pump electrode 12 is electrically connected, throughwiring line 63, with the third connection terminal 19 of electroniccontrol unit 5. Second pump electrode 16 is electrically connected,through wiring line 62, with the second connection terminal 17 ofelectronic control unit 5. First pump electrode 12 is covered with aporous protective layer 29 protecting first pump electrode 12 againstpoisonous substance.

The oxygen concentration sensing cell 24 includes a plate-shaped solidelectrolyte member 23, and first and second sensing electrodes 22 and 28formed, respectively, on front and rear surfaces of solid electrolytemember 23. The solid electrolyte member 23 is made of an oxygen ionconductive solid electrolyte. In this example, solid electrolyte member23 is made of partially stabilized zirconia (ZrO₂). First and secondsensing electrodes 22 and 28 are made of material containing platinum,as main component. First sensing electrode 22 is electrically connected,through wiring line 62, with the second connection terminal 17 ofelectronic control unit 5, and further connected with second pumpelectrode 16. Second sensing electrode 28 is electrically connected,through wiring line 61, with the first connection terminal 15 ofelectronic control unit 5.

A porous diffusion layer 18 is provided in a part of an insulating layer(not shown) which is formed between oxygen pump cell 14 and oxygenconcentration sensing cell 24, and which is made mainly of an insulatingmaterial (such as alumina) for electrically insulating both cells 14 and24 from each other. Porous diffusion layer 18 is a porous layer mademainly of an insulating material (such as alumina) for controllingdiffusion of measurement gas introduced into sensing element 10. Insteadof porous diffusion layer 18, it is optional to form small hole or holesin a side wall of the above-mentioned insulating layer, as a diffusioncontrol section.

A measurement chamber 20 is a hollow portion surrounded by the porousdiffusion layer 18 and the above-mentioned insulating layer, betweenoxygen pump cell 14 and oxygen concentration sensing cell 24. Thismeasurement chamber 20 is in communication with a measurement gasatmosphere through porous diffusion layer 18 (or porous section). Thesecond pump electrode 16 is bared in an upper portion of measurementchamber 20, and the first sensing electrode 22 is bared in a lowerportion of measurement chamber 20.

The reinforce plate 30 is attached to the rear side surface of oxygenconcentration sensing cell 24, opposite to the front side surface facingto the measurement chamber 20, so as to form a laminate to increase thestrength of sensor element 10 as a whole. Reinforce plate 30 is made ofa material containing, as a main component, ceramic, and shaped in theform of plate approximately equal in size to the solid electrolytemembers 13 and 23.

The second sensing electrode 28 is sandwiched between the reinforceplate 30 and oxygen ion conductive solid electrolyte member 23, andshielded from the outside. A reference oxygen chamber 26 is formed inthe form of a sealed space, around the second sensing electrode 28.Therefore, it is possible to accumulate oxygen at a substantiallyconstant concentration in the reference oxygen chamber 26 by supplying aminute constant current Icp in a direction from second sensing electrode28 toward first sensing electrode 22 and pumping oxygen from measurementchamber 20 toward second sensing electrode 28. Thus, the oxygenconcentration in reference oxygen chamber 26 is used as a referenceoxygen concentration in sensing the actual oxygen concentration.

The heater 43 in the form of a flat plate is disposed so as to confrontthe oxygen pump cell 14 of sensing element 10. Heater 43 is made of amaterial containing alumina as a main component, and includes therein aheater wire 72 made of a material containing platinum as a maincomponent. Heater 43 is controlled by electric power supplied fromheater control circuit 60 so as to control the temperature of sensingelement 10 to the target activation temperature (830° C., in thisexample). One end of the heater wire 72 is electrically connected withheater control circuit 60. The other end of heater wire 72 is connectedwith a battery VB (12V battery in this example). By heating, the heater43 activates the sensing element 10 (oxygen pump cell 14 and oxygenconcentration sensing cell 24), and enables a gas sensing (oxygensensing) action.

The thus-constructed gas sensor 8 (sensing element 10) is operated inthe following manner. First, the measurement gas (exhaust gas in thisexample) diffuses through porous diffusion layer 18 into measurementchamber 20. If then air fuel mixture supplied to the engine (or themeasurement gas in measurement chamber 20) is held at the theoreticalair fuel ratio, an electromotive force of 450 [mV] is produced in oxygenconcentration sensing cell 24 by an oxygen concentration differencebetween the measurement chamber 20 and reference oxygen chamber 26providing the reference oxygen concentration. (A potential difference of450 [mV] is produced between the first and second sensing electrodes 22and 28.)

In dependence on variation in the air fuel ratio of the air fuel mixturesupplied to the engine, the concentration of oxygen contained in theexhaust gas varies, and hence the oxygen concentration of themeasurement gas in measurement chamber 20 is varied. Therefore, theengine control system 1 of this example controls the Ip current flowingthrough the oxygen pump cell 14 with the sensor control circuit 2 so asto hold the potential difference between the first and second sensingelectrodes 22 and 28 at 450 [mV]. Thus, by controlling the Ip current soas to control the atmosphere in measurement chamber 20 at the samecondition as the theoretical air fuel ratio, the control system causesthe oxygen pump cell 14 to perform an oxygen pumping action.

The oxygen pump cell 14 is arranged to be switched between an operationof drawing oxygen from measurement chamber 20 and an operation ofdelivering oxygen into measurement chamber 20 in response to thedirection of current supplied between the electrodes (first and secondpump electrodes 12 and 16). Moreover, the oxygen pump cell 14 is capableof adjusting the pumping quantity of oxygen in accordance with themagnitude of the current supplied between the electrodes. Therefore,engine control device 9 can calculate the oxygen concentration of themeasurement gas in accordance with the current condition (such as thecurrent direction and the current magnitude) of the Ip current.

FIG. 2 is a circuit diagram schematically showing the electronic controlunit 5, for explaining the construction and operations of electroniccontrol unit 5. As mentioned before, the electronic control unit 5includes the sensor control circuit 2, heater control circuit 60 andengine control device 9. The sensor control circuit 2 includes thesensor drive circuit 52 and control section 55.

The heater control circuit 60 includes a transistor Tr including acollector connected with one end of the heating resistive element 72, anemitter grounded through a resistor Rh, and a base connected with theengine control device 9. Therefore, by outputting a signal of a voltagelevel turning on the transistor Tr (a heater on signal or “command”) tothe base of transistor Tr, the engine control device can cause a currentto flow through the heating resistive element 72 by supplying a voltagefrom the battery VB, and thereby generate heat with the heater 43. Whenthe engine control device 9 stops outputting the heater on signal, thetransistor Tr turns off, and stops supplying current to the heatingresistive element 72, so that the heating operation of heater 43 isterminated.

In this example, the voltage applied to heater 43 is controlled bysetting the duty ratio (or duty cycle) to 100% when the maximumeffective voltage (=the source voltage of battery VB which is 12V inthis example) is to be applied to heater 43, and by varying the dutyratio between 0%˜100% when a voltage lower than the maximum effectivevoltage is applied. For this control, the engine control device 9delivers the above-mentioned heater on signal determined by the dutyratio, to heater control circuit 60, and the heater control circuit 60is operated in an on-off mode in response to the heater on signal. Thus,this system controls the current supply to heater 43 by PWM.

The sensor drive circuit 52 has the following circuit configurationmainly for measuring the oxygen concentration. Sensor drive circuit 52includes an operational amplifier OP2, a PID control circuit 69 and aconstant current circuit 62. Operational amplifier OP2 is for flowingthe Ip current to drive the oxygen pump cell 14. PID control circuit 69is a circuit for improving the control characteristic of Ip current.Constant current circuit 62 is a circuit for supplying the constantcurrent Icp to oxygen concentration sensing cell 24 to hold constant theoxygen concentration around the second sensing electrode 28 (in thereference oxygen chamber 26).

Sensor drive circuit 52 further includes switches SW1˜SW3 for changingthe operation state of sensor drive circuit 52 in accordance with switchcommand signals outputted from engine control device 9. In accordancewith changes in the on/off states of switches SW1˜SW3, the sensor drivecircuit 52 can perform an operation to supply the sensor signal to sensethe impedance signal Vrpvs, an operation to sense the impedance signalVrpvs, and various other operations to control the current supply toheater 43 and to sense the temperature of gas sensor 8.

Moreover, sensor drive circuit 52 includes a Vcent point connected withthe COM terminal 17. The output terminal of PID control circuit 69, theinverting input terminal of operational amplifier OP2, and one of inputterminals of differential amplifier circuit 61 are connected to theVcent point. The output terminal of PID control circuit 69 is connectedthrough a sensing resistor R1, to the Vcent point. A reference voltageof 3.6V is applied to the non-inverting input terminal of operationalamplifier OP2. The output terminal of operational amplifier OP2 isconnected to the Ip+ terminal. Thus, operational amplifier OP2 forms apart of a negative feedback circuit for controlling the current supplyto sensing element 10 (oxygen pump cell 14, to be exact).

The other input terminal of differential amplifier circuit 61 isconnected with a junction point (Vpid point) between the output terminalof PID control circuit 69 and the sensing resistor R1. The differentialamplifier circuit 61 is configured to amplify a voltage differencebetween both ends of sensing resistor R1. The second pump electrode 16is connected to the Vcent point through resistor R, wiring line 62 andsecond connection terminal (COM terminal) 17.

The PID control circuit 69 performs a PID calculation of a deviation ΔVsbetween the control target voltage of 450 mV for oxygen concentrationsensing cell 24, and the output voltage Vs of oxygen concentrationsensing cell 24 and thereby performs the function of the negativefeedback control to improve the control characteristic. The inputterminal of PID control circuit 69 is connected through an operationalamplifier OP1, with the output terminal of an operational amplifier OP4whose non-inverting input terminal is connected with the Vs+ terminal15. Thus, the output voltage Vs of oxygen concentration sensing cell 24is inputted, through operational amplifier OP4, into PID control circuit69. The inverting input terminal of operational amplifier OP4 isconnected with its own output terminal.

The constant current circuit 62 connected with the Vs+ terminal 15includes a resistor connected in series with a constant voltage source(denoted by 8V in FIG. 2). Constant current circuit 62 is arranged tosupply the constant current Icp (of 17 μA, for example) to oxygenconcentration sensing cell 24 to hold the oxygen concentration constantaround second sensing electrode 28 of oxygen concentration sensing cell24 (in reference oxygen chamber 26).

These circuits are arranged to sense the oxygen concentration in thefollowing manner. First, the sensor drive circuit 52 supplies the minuteconstant current Icp to oxygen concentration sensing cell 24 withconstant current circuit 62, and simultaneously controls the pumpcurrent Ip flowing through oxygen pump cell 14 so as to control thevoltage Vs between both ends of oxygen concentration sensing cell 24(between the Vs+ terminal and the COM terminal) to 450 mV. By so doing,the sensor drive circuit 52 draws oxygen into and out of the measurementchamber 20. That is, sensor drive circuit 52 adjusts the oxygenconcentration (oxygen partial pressure) in measurement chamber 20 withthe oxygen pump cell 14 so as to bring the voltage Vs across the oxygenconcentration sensing cell 24 to 450 mV.

The magnitude and direction of pump current Ip flowing through oxygenpump cell 14 are varied in dependence on the oxygen concentration (airfuel ratio) of the exhaust gas. Therefore, it is possible to calculatethe oxygen concentration in the exhaust gas by sensing this pump currentIp with sensing resistor R1 and by using the gas sensor signal Vipobtained by the differential amplification of the voltage across thissensing resistor R1. The reference oxygen chamber 26 can be used as aninternal oxygen reference source by the flow of minute current Icpthrough oxygen concentration sensing cell 24 in the direction to drawoxygen from measurement chamber 20 toward porous electrode 28.

PID control circuit 69 is configured to control the pump current Ip inthe PID control mode to control the potential difference to 450 mVbetween the potential at the Vs+ terminal of oxygen concentrationsensing cell 24 connected through first operational amplifier OP1 andthe potential at the COM terminal (Vcent point). In this example, theresult of the PID calculation of PID control circuit 69 based on thedeviation between the target control voltage (450 mV) and the voltage Vsacross oxygen concentration sensing cell 24 is fed back to secondoperational amplifier OP2, and the second operational amplifier OP2supplies the pump current Ip to oxygen pump cell 14.

The sensing resistor R1 is arranged to sense the magnitude of pumpcurrent Ip and to convert into a voltage signal. The voltage betweenboth ends of this sensing resistor R1 (the difference between thepotential at the Vcent point and the potential at the Vpid point) isdifferential-amplified with a predetermined amplification degree by thedifferential amplifier 61, and supplied as the gas sensor signal Vip,from a signal output terminal 43 to engine control device 9.

Engine control device 9 converts the gas sensor signal Vip to a digitalsignal with an A/D converter circuit (not shown). Thereafter, enginecontrol device 9 performs the concentration calculating process tocalculate the oxygen concentration corresponding to the gas sensorsignal Vip by using a functional relationship in the form of map (ormaps) or mathematical expression (or expressions).

Sensor drive circuit 52 has the following circuit configuration mainlyused for sensing the impedance Rvps of oxygen concentration sensing cell24.

The non-inverting input terminal of first operational amplifier OP1 isconnected, through a first capacitor C1 and the first switch SW1, withthe output terminal of fourth operational amplifier OP4. Firstoperational amplifier OP1, first capacitor C1 and first switch SW1 forma sample and hold circuit. This sample hold circuit turns the firstswitch SW1 from ON to OFF at the time of impedance sensing of oxygenconcentration sensing cell 24 and thereby hold the voltage Vs (thepotential at the Vs+ terminal) produced across the oxygen concentrationsensing cell 24 just before the current supply for sensing the impedanceof oxygen concentration sensing cell 24. By so doing, the sample holdcircuit inputs the voltage Vs just before the detection of theimpedance, to the PID control circuit 69.

One end of first capacitor C1 is connected with the junction pointbetween first switch SW1 and the non-inverting input terminal of firstoperational amplifier OP1. The other end of first capacitor C1 isgrounded.

The output terminal of first operational amplifier OP1 is connected,through a resistor R3, with the non-inverting input terminal (+terminal) of third operational amplifier OP3. Moreover, one end ofresistor R5 is connected with the junction point between the resistor R3and the non-inverting input terminal of third operational amplifier OP3.The other end of resistor R5 is grounded.

The inverting input terminal of third operational amplifier OP3 isconnected, through a resistor R4, with the output terminal of fourthoperational amplifier OP4, and further connected with the outputterminal of third operational amplifier OP3 itself through a resistorR6. Therefore, the inverting input terminal of third operationalamplifier OP3 receives the Vs+ potential (the potential at the Vs+terminal) at the time of supply of a later-mentioned current −Iconst forsensing the impedance, to oxygen concentration sensing cell 24.

Third operational amplifier OP3 outputs a voltage variation ΔVscorresponding to a difference between the hold value held by firstoperational amplifier OP1 (the voltage Vs of oxygen concentrationsensing cell 24 immediately before the supply of current for sensing theimpedance) and the Vs+ potential at the time of supply of the impedancesensing current −Iconst to oxygen concentration sensing cell 24 (theoutput potential of fourth operational amplifier OP4). Since thisvoltage variation ΔVs is approximately proportional to a bulk resistanceof oxygen concentration sensing cell 24, this voltage variation ΔVs canbe used as the impedance signal Vrpvs representing the impedance ofoxygen concentration sensing cell 24. Thus, third operational amplifierOP3 outputs the voltage variation ΔVs and outputs the impedance signalVrpvs approximately proportional to the bulk resistance of oxygenconcentration sensing cell 24.

In this sensor control device 5, the third operational amplifier OP3 andresistors R3˜R6 form a differential amplifying type operationalamplifier circuit (that is, differential amplifier circuit) 90. Thus,the operational amplifier circuit 90 for sensing the impedance is of thedifferential amplifying type which can remove noises inputted to the twoinput terminals in common, and therefore, the sensor control device 5can produce the adequate impedance signal Vrpvs receiving littleinfluence from noises.

The output terminal of operational amplifier 90 (third operationalamplifier OP3) is connected, through second switch SW2 and resistor R2,with the non-inverting input terminal of fifth operational amplifierOP5. The inverting input terminal of fifth operational amplifier OP5 isconnected with the output terminal of fifth operational amplifier OP5itself, to receive the output of fifth operational amplifier OP5. Oneend of second capacitor C2 is connected with the junction point betweenresistor R2 and the non-inverting input terminal of fifth operationalamplifier OP5. The other end of second capacitor C2 is grounded. Theoutput terminal of fifth operational amplifier OP5 is connected, throughthe signal output terminal 41, with the engine control device 9.

The fifth operational amplifier OP5, second capacitor C2, second switchSW2 and resistor R2 form a signal hold circuit. The voltage variationΔVs is inputted from third operational amplifier OP3 to this signal holdcircuit when second switch SW2 is turned from OFF to ON at the time ofsensing the impedance of oxygen concentration sensing cell 24.Thereafter, when second switch SW2 is turned from ON to OFF, the signalhold circuit retains the voltage variation ΔVs outputted from thirdoperational amplifier OP3 at the time of the on state of second switchSW2, with second capacitor C2, and delivers impedance signal Vrpvsrepresenting the voltage variation ΔVs to engine control device 9.

Then, engine control device 9 converts the impedance signal Vrpvs to thedigital signal with the A/D converter (not shown), and thereafterperforms the temperature sensing process of calculating the impedance ofoxygen concentration sensing cell 24, and hence the temperature of gassensor 8, by using the map or mathematical expression retained in enginecontrol device 9.

A current source 63 is connected, through a second switch SW2, with theVs+ terminal 15. A current source 65 is connected, through anothersecond switch SW2, with the junction point between the resistor R andCOM terminal 17. These current sources 63 and 65 are a source forsupplying the constant current −Iconst to sense the impedance of oxygenconcentration sensing cell 24.

Similarly, a current source 64 is connected, through third switch SW3,with the Vs+ terminal 15. A current source 66 is connected, throughanother third switch SW3, with the junction point between the resistor Rand COM terminal 17. These current sources 64 and 66 are a source forsupplying the constant current +Iconst which is opposite in polarity tothe impedance sensing constant current −Iconst.

The (switch) control section 55 is a circuit for controlling theswitching states (on/off states) of these first, second and thirdswitches SW1˜SW3. After receipt of a notification signal Sr signaling astart of the temperature sensing process from engine control device 9,the control section 55 is operated to control the states of the switchesSW1˜SW3 according to a timer count of a timer circuit provided incontrol section 55.

The first switch SW1 is arranged to control a voltage holding operationof first operational amplifier OP1 (sample hold circuit). Secondswitches SW2 are arranged to control the on/off state of the constantcurrent −Iconst for sensing the impedance of oxygen concentrationsensing cell 24, and to control the signal hold operation of fifthoperational amplifier OP5 (the signal hold circuit). The third switchesSW3 are arranged to control the on/off state of the constant current+Iconst opposite to the constant current −Iconst in polarity.

FIG. 3 shows a main characteristic portion of this embodiment accordingto the present invention, in the form of a graph illustrating theoverheat preventing process performed by engine control device 9. Inthis embodiment, if the impedance Rpvs of oxygen concentration sensingcell 24 becomes greater than or equal to an abnormality judgingthreshold (threshold value of the impedance for judging abnormality)because of a trouble such as a disconnection of the Vs+ line of oxygenconcentration sensing cell 24, the control system (i) terminates theapplication of the maximum effective voltage to heater 43 at the timingwhen the application time becomes equal to a predetermined overheatpreventing time (or time period), and (ii) supplies a current to theheater 43 with a low (or lower) effective voltage which is lower thanthe maximum effective voltage and which is adequate for making thetemperature of oxygen concentration sensing cell 24 higher than or equalto 500° C. (or making the impedance Rvps of oxygen concentration sensingcell 24 equal to a value corresponding a temperature higher than orequal to 500° C.).

When the Vs+ line of oxygen concentration sensing cell 24 is broken, andhence the impedance becomes excessively or infinitely great, the controlsystem misjudges that the temperature of oxygen concentration sensingcell 24 is too low, and continues the application of the maximumeffective voltage to heater 43, as shown in FIG. 3. As a result, thetemperature of gas sensor 8 might be increased abnormally (beyond 1000°C., as shown by a one dot chain line in FIG. 3), resulting in breakage.

If the Vs+ line is short-circuited with the ground potential, thecontrol system is unable to detect the signal itself. Therefore, thecontrol system can detect the occurrence of an abnormal conditionimmediately, and responds to the abnormal condition adequately accordingto another process.

Therefore, when the impedance Rpvs of oxygen concentration sensing cell24 becomes greater than or equal to the abnormality judging threshold(400Ω, in this example), the control system of this embodiment preventsoverheat of gas sensor 8 (as shown by a solid line in FIG. 3) bylimiting the application time (duration) of the maximum effectivevoltage to heater 43 to the predetermined heater overheat preventingtime (Top, Tcl).

After the expiration of the heater overheat preventing time, the controlsystem applies the lower effective voltage lower than the maximumeffective voltage to heater 43. Thus, instead of cutting off the currentsupply to heater 43 in response to the occurrence of the abnormalcondition, the control system continues the heating operation with thelower voltage to hold a temperature for preventing the gas sensor 8(sensing element 10) from becoming cool and thereby preventing foreignmatter (such as carbon) from adhering to the surface of sensing element10.

When the impedance Rpvs of oxygen concentration sensing cell 24 becomeslower than the abnormality judging threshold, the control system resumesthe normal heater control operation based on the PI calculation. By thisPI calculation which is known per se, the control system calculates theheater application voltage to hold the temperature of gas sensor 8constant, from the difference ΔRpvs between the target value ofimpedance Rpvs and the value of impedance Rpvs at the time of operationof a later-mentioned step S18.

In this example, the lower effective voltage is set at a voltage (8V inthis example) for controlling the temperature of oxygen concentrationsensing cell 24 equal to or higher than the predetermined temperature(500° C. in this example) which is effective for preventing adherence ofundesired substance such as carbon in the exhaust gas onto the surfaceof sensing element 10. If the lower effective voltage is set at such alevel as to hold the temperature of oxygen concentration sensing cell 24higher than or equal to 700° C., then it is possible to preventadherence of substances such as phosphorous, silicon and lead. Apreferable range of the temperature of oxygen concentration sensing cell24 is 750° C.˜900° C., and preferably, the lower effective voltage isset at such a level to hold the temperature of oxygen concentrationsensing cell 24 within this temperature range. It is possible to measurethe temperature of oxygen concentration sensing cell 24 from theimpedance Rpvs of oxygen concentration sensing cell 24.

The heater overheat preventing time can be determined experimentally onthe basis of the results of experiment for measuring an amount of timeuntil the temperature is increased to 1000° C. by the continuation ofapplication of the maximum effective voltage. The length of theallowable heater overheat preventing time is varied in dependence on thecondition (such as the temperature) of gas sensor 8 before theapplication of the maximum effective voltage. Accordingly, this exampleemploys a time (amount of time) Top from a start of application of themaximum effective voltage in the state in which the gas sensor 8 is atnormal temperature, until the temperature of gas sensor 8 becomes equalto 1000° C. (second continuation value or second amount of time), and atime (amount of time) Tcl from a start of application of the maximumeffective voltage in the state in which gas sensor 8 is at a targetoperating temperature (temperature at the P1 control, 830° C.), untilthe temperature of gas sensor 8 becomes equal to 1000° C. (firstcontinuation value or first amount of time). Bt setting the heateroverheat preventing time in this way, the control system can preventoverheating of gas sensor 8 more accurately. The second continuationvalue (Top) is longer than the first continuation value (Tcl). In thisexample, the impedance Rpvs is about 75Ω when gas sensor 8 is at thetarget operating temperature (830° C., in this example).

FIGS. 4 and 5 show the heater control process and the heater overheatpreventing process performed by engine control device 9. In thisexample, the heater control process shown in FIG. 4 is performed as amain routine, and the heater overheat preventing process shown in FIG. 5is performed as a subroutine. Simultaneously with the heater controlprocess, the engine control device 9 performs the temperature sensingprocess for sensing the temperature of gas sensor 8. The temperaturesensing process is known per se. In brief, engine control device 9obtains the impedance signal Vrpvs outputted from sensor drive circuit52 at a predetermined sensing timing. Then, from impedance signal Vrpvs,the engine control device 9 calculate the impedance Rpvs of oxygenconcentration sensing cell 24 by using a predetermined mathematicalexpression or data (such as a two dimensional map) representing arelationship between the impedance signal Vrpvs and the impedance Rpvs.Then, engine control device 9 calculates the temperature of oxygenconcentration sensing cell 24 (or the temperature of gas sensor 8) inaccordance with the impedance Rpvs. The thus-calculated temperature isused for the heater control process.

At the time of sensing impedance Rpvs, the system supplies the constantcurrent −Iconst to oxygen concentration sensing cell 24 by turning onsecond switches SW2. After sensing the impedance, the system suppliesthe opposite polarity current +Iconst opposite in polarity to theimpedance sensing current (−Iconst), to oxygen concentration sensingcell 24 by turning on third switches SW3. By so doing, the system canreduce a recovery time until a restoration of a normal state from astate in which the solid electrolyte member of oxygen concentrationsensing cell 24 is oriented by the impedance sensing current, and theinternal electromotive force is affected (the state in which theoutputted electromotive force does not reflect the oxygen concentrationdifference correctly). The process for sensing impedance Rpvs of oxygenconcentration sensing cell 24 is performed at regular time intervals of100 msec, for example under the control of control section 55.

FIG. 4 shows the heater control process performed as the main routine.After a start of the heater control process, engine control device 9first performs an initializing operation at a step S10. In thisoperation, engine control device 9 resets an overheat prevention flagand a sensor activation experience flag and an overheat preventioncounter, to zero. The overheat prevention flag is a condition code fortransferring control immediately to the overheat preventing process whenthe overheat prevention flag is on (=1). The sensor activationexperience flag is a condition code representing the state in which gassensor 8 is activated, and gas sensor 8 is in a normal gas measuringstate (allowing the flow of the pump current Ip). The sensor activationexperience flag is on (=1) when gas sensor 8 is activated. The overheatprevention counter is used for counting or measuring the heater overheatpreventing time (Top, Tc1) shown in FIG. 3.

From S10, the engine control device 9 proceeds to a step S12, and readsthe impedance Rpvs outputted from sensor drive circuit 52. Then, at astep S14, engine control device 9 examines whether the overheatprevention flag is ON (equal to one) or not. When the overheatprevention flag is ON, the engine control device 9 proceeds from S14 toa step S25 directly and starts the subroutine (the overheat preventingprocess of FIG. 5). When the overheat prevention flag is OFF, and hencethe answer of S14 is NO, then engine control device 9 proceeds to a stepS16.

At S16, engine control device 9 examines whether the sensor activationexperience flag is ON (one) or not. From S16, the engine control device9 proceeds to a step S18 when the sensor activation experience flag isON and the answer of S16 is YES, and to a step 20 when it is not and theanswer of S16 is NO.

At step S18, engine control device 9 calculates the heater applicationvoltage (Vhrms) by the PI calculation. The PI calculation is known perse, as mentioned before, and the heater application voltage iscalculated so as to hold the temperature of gas sensor 8 constant. Inaccordance with the thus-calculated heater application voltage, theengine control device 9 controls the supply of electricity to heater 43through heater control circuit 60 (in the manner of the PWM control).

At step S20, engine control device 9 examines whether the impedance Rpvsobtained at S12 is smaller than a sensor activation judgment or judgingthreshold (which is set to 400Ω in this example). When the impedanceRpvs is smaller than the activation judging threshold (400Ω), the enginecontrol device 9 considers that gas sensor 8 is in the normal activestate, and sets the sensor activation experience flag to ON (one) at astep S22. From S22, engine control device 9 proceeds to step S18.

When the impedance Rpvs is greater than or equal to the activationjudging threshold, the engine control device 9 proceeds from S20 to astep S24, and sets the application voltage (Vhrms) to heater 43 to themaximum effective voltage at S24. At the stage of S24, it is uncertainwhether the impedance Rpvs is not normal because of trouble such as linedisconnection, or merely the temperature of gas sensor 8 is low.

From S18 or S24, the engine control device 9 proceeds to step S25 andstarts the heater overheat preventing process of FIG. 5 as thesubroutine. When the subroutine of the heater overheat preventingprocess is finished, the engine control device 9 proceeds to a step S26,and examines, at S26, whether a predetermined impedance Rpvs readingcycle (10 msec in this example) of S12 has elapsed. Then, engine controldevice 9 returns to S12 when the answer of S12 becomes YES after theelapse of the Rpvs reading cycle, and waits when the answer of S12 isNO.

FIG. 5 shows the overheat preventing process performed as thesubroutine. After a start of the overheat preventing process, enginecontrol device 9 first examines whether the impedance Rpvs of oxygenconcentration sensing cell 24 is smaller than the predeterminedabnormality judgment or judging threshold (which is set equal to 400Ω inthis example). When the impedance Rpvs is smaller than the abnormalityjudgment threshold (400Ω), then engine control device 9 proceeds fromS50 to a step S52. At S52, engine control device 9 resets the overheatprevention counter to zero, and resets the overheat prevention flag toOFF (zero). After S52, engine control device 9 terminates the process ofFIG. 5.

When impedance Rpvs is greater than or equal to the abnormality judgmentthreshold (400Ω) and hence the answer of S50 is NO, then engine controldevice 9 examines, at a step S54, whether the heater application voltage(Vhrms) is equal to the maximum effective voltage or not. If the answerof 554 is NO, the engine control device 9 terminates the process of FIG.5.

In the case of YES of S54, the engine control device 9 judges that theheater application voltage is increased to the maximum effective voltagewhile the abnormal condition of S50 is met, and proceeds to a step S56.At S56, engine control device 9 increments the overheat preventioncounter to start measuring time to stop the continuation of theapplication of the maximum effective voltage at the expiration of theoverheat preventing time as shown in FIG. 3.

Then, engine control device 9 examines whether the sensor activationexperience flag is ON (one) or not at a step S58. The judgment of S58 isperformed to estimate the temperature condition of gas sensor 8 bychecking the activation experience flag, and thereby to select asuitable amount of the heater overheat preventing time (Top, Tcl) inaccordance with the temperature of gas sensor 8.

When the sensor activation experience flag is OFF and hence the answerof S58 is NO, then engine control device 9 proceeds to a step S60. AtS60, engine control system 9 assumes that gas sensor 8 is not yetactivated and still cold, and examines whether the overheat preventioncounter is greater than or equal to an amount of time or interval Top.

When the overheat prevention counter is greater than or equal to Top andhence the answer of S60 is YES, the engine control device 9 judges thatthe application of the maximum effective voltage is continued for a timeduration longer than or equal to Top, and proceeds to a step S62. AtS62, engine control device 9 delivers a command signal (in the form ofthe heater on signal) to decrease the heater application voltage (Vhrms)to the low effective voltage (lower effective voltage), to heatercontrol circuit 60, and sets the heater overheat prevention flag to one.In this example, this command signal commands a decrease of the heaterapplication voltage from the maximum effective voltage to the lowereffective voltage (8V) by decreasing the duty cycle from a greater valuecorresponding to the maximum effective voltage to a lower valuecorresponding to the lower effective voltage. In response to thiscommand signal in the form of the heater on signal, the heater controlcircuit 60 applies the lower effective voltage to heater 43. After S62,engine control device 9 terminates the overheat preventing process ofFIG. 5 and returns to the main routine of FIG. 4.

The overheat prevention flag is set to one at S62. Therefore, whenengine control device 9 returns to the main routine of FIG. 4, theengine control device 9 can proceed from S14 directly to S25, andcontinue the heater overheat preventing process of FIG. 5. When step S62is reached, there is a possibility that the abnormality such as thedisconnection of Vs+ line continues to be present. Therefore, theoverheat prevention flag is set to one at 562.

When the sensor activation experience flag is ON and hence the answer ofS58 is YES, then engine control device 9 proceeds to a step S64. At 564,engine control system 9 assumes that gas sensor 8 is already activatedand controlled in the PI control mode, and the temperature of gas sensor8 is increased to the operating temperature (830° C., for example), andexamines whether the overheat prevention counter is greater than orequal to an amount of time or interval Tcl.

When the overheat prevention counter is greater than or equal to Tcl andhence the answer of S64 is YES, the engine control device 9 judges thatthe application of the maximum effective voltage is continued for a timeduration longer than or equal to Tcl, and proceeds to a step S66. AtS66, engine control device 9 delivers the command signal (in the form ofthe heater on signal) to decrease the heater application voltage (Vhrms)to the low effective voltage (lower effective voltage), to heatercontrol circuit 60, and sets the heater overheat prevention flag to one.In this example, this command signal commands the decrease of the heaterapplication voltage from the maximum effective voltage to the lowereffective voltage (8V) by decreasing the duty cycle from the greatervalue corresponding to the maximum effective voltage to the lower valuecorresponding to the lower effective voltage. In response to thiscommand signal in the form of the heater on signal, the heater controlcircuit 60 applies the lower effective voltage to heater 43. After S66,engine control device 9 terminates the overheat preventing process ofFIG. 5 and returns to the main routine of FIG. 4. At S66, the overheatprevention flag is set to one for the same reason as in step S62.

In this embodiment, step S12 corresponds to “impedance sensing step”,and step S50 corresponds to “impedance condition examining step” (or“impedance abnormality judging step”). Step S54 corresponds to “voltagecondition examining step” (or “voltage application judging step”). StepsS56, S58, S60 and S64 correspond to “duration measuring step” (or“voltage application condition judging step”). The operations of enginecontrol device 9 and/or heater control circuit 60 according to steps S62and S64 correspond to “voltage decreasing step” (or “lower effectivevoltage applying step”). Step S20 corresponds to “activation judgingstep”. The operation of setting the amount of the heater overheatpreventing time in dependence on the judgment of S58 corresponds to“time setting step”.

The present invention is not limited to the above-mentioned embodiment.Various variations and modifications are possible within the purview ofthe present invention. For example, the sensing element 10 may be any ofvarious types besides the full range air fuel ratio employed in theillustrated example. The present invention is applicable to a gassensor, such as a NOx sensor, in which another cell is added to sensingelement, and there are formed two of measurement chambers. In theillustrated embodiment, the gas sensor control apparatus is incorporatedin ECU 5. However, it is optional to provide the gas sensor controlapparatus separate from ECU 5. For example, the gas sensor controlapparatus is provided between the gas sensor 8 and ECU5, and constructedto include the sensor control circuit 2, the heater control circuit 60and a microcomputer capable of executing the heater control process andthe heater overheat preventing process. The microcomputer may be formedon or in a circuit board including the microcomputer alone or togetherwith the circuits 2 and 60. In the illustrated example, the activationjudging threshold used in step S20 and the abnormality judging thresholdused in step S50 are equal to each other. However, it is possible to setthe activation judging threshold and the abnormality judging thresholdat two different amounts, respectively.

According to the illustrated embodiment, a gas sensor control apparatusis adapted to be connected with a gas sensor for sensing theconcentration of a specified gas, or the gas sensor control apparatusincludes a connecting section (which may include one or more terminals)adapted to be connect with the gas sensor. Alternatively, the gas sensorcontrol apparatus may be a gas sensor control system including the gassensor. The gas sensor may be a sensor including at least one cell(monitored cell) including a solid electrolyte member and a pair ofelectrodes separated from each other by the solid electrolyte member.The gas sensor control apparatus comprises: a heater regulating sectionto control the supply of electricity to a heater included in the gassensor; an impedance sensing section to sense an impedance of the(monitored) cell of the gas sensor, from an output of the cell; a sensor(impedance) condition examining section to examine whether the impedancesensed by the impedance sensing section is greater than or equal to apredetermined abnormality judging threshold; a heater (voltage)condition examining section to examine whether a maximum effectivevoltage (or a higher effective voltage higher than or equal to apredetermined level which may be equal to the maximum effective voltageas in the illustrated, example) is applied to the heater, when theimpedance is judged to be greater than or equal to the predeterminedabnormality judging threshold; a time duration measuring section toexamine whether an application time duration of application of themaximum effective voltage (or the higher effective voltage) to theheater becomes equal to or longer than a predetermined heater overheatpreventing time; and a voltage decreasing section to command the heaterregulating section to decrease the voltage applied to the heater to alower effective voltage and continue the application of the voltage tothe heater at the lower effective voltage which is lower than themaximum effective voltage and which is so set as to hold the temperatureof the cell higher than or equal to a predetermined temperature valuesuch as 500° C. when the application time duration becomes equal to orlonger than the predetermined heater overheat preventing time. It ispossible to determine whether the lower effective voltage to hold thetemperature of the monitored cell higher than or equal to 500° C. isapplied to the heater or not, by examining the impedance sensed throughthe monitored cell to determine whether the sensed impedance is at avalue which can be assumed when the temperature is equal to or higherthan 500° C.

According to one of various possible interpretations of the embodiment,a gas sensor control apparatus has a basic construction which comprises:a heater regulating section to supply an electric power or electricityto a heater included in a gas sensor for sensing a concentration of aspecified gas; an impedance sensing section to sense an impedance of a(monitored) cell of the gas sensor; and a controlling section which isconfigured to check a first condition (S50: NO) which is satisfied whenthe impedance is greater than or equal to a predetermined abnormalityjudging threshold, to check a second condition (S54: YES) which issatisfied when the electric power supplied to the heater is higher thanor equal to a predetermined higher power level (such as the maximumeffective voltage), to measure a continuance (or time duration) of athird condition which is satisfied when the first and second conditionsare both satisfied simultaneously (S56), and to decrease the electricpower (or the quantity of electricity) supplied to the heater by theheater regulating section, to a lower power level lower than the higherpower level when the continuance of the third condition reaches apredetermined overheat preventing time. The thus-constructed gas sensorcontrol apparatus may further comprise any one or more of the followingfeatures.

(1) The controlling section of the basic construction may be configuredto adjust the predetermined overheat preventing time in accordance witha starting (temperature) state of the gas sensor before or just before astart of the continuance of the third condition. (2) The lower powerlevel may be so set as to hold the temperature of the gas sensor at sucha higher temperature level preventing adherence of a substance, such ascarbon, contained in a measurement gas to the gas sensor. (3) Thecontrolling section may be configured to control the electric powersupplied to the heater with the heater regulating section normally in anormal heater control mode (such as PI control mode at S16) inaccordance with the impedance of the (monitored) cell of the gas sensor(for example, so as to decrease a deviation between the sensed actualimpedance and a desired target impedance). (4) The controlling sectionmay be configured to terminate the normal heater control mode andinstead to start a limiting heater control mode (including a limitingtime mode until the end of the overheat preventing time and a limitingpower mode with the lower power level after the end of the overheatpreventing time) when the first and second conditions become satisfied.(5) The controlling section may be configured to terminate the limitingheater control mode and instead resumes the normal heater control modewhen the first condition becomes unsatisfied.

This application is based on a prior Japanese Patent Application No.2008-285054 filed on Nov. 6, 2008. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

What is claimed is:
 1. A gas sensor control apparatus for controlling agas sensor for sensing a concentration of a specified gas, the gassensor including a cell including a solid electrolyte member and a pairof electrodes formed on the solid electrolyte member, the gas sensorcontrol apparatus comprising: a heater regulating section to control asupply of electricity to a heater included in the gas sensor; animpedance sensing section to sense an impedance of the cell of the gassensor, from an output of the cell; an impedance condition examiningsection to examine whether the impedance sensed by the impedance sensingsection is greater than or equal to a predetermined abnormality judgingthreshold; a voltage condition examining section to examine whether amaximum effective voltage is applied to the heater, when the impedanceis judged to be greater than or equal to the predetermined abnormalityjudging threshold; a duration measuring section to examine whether anapplication time duration of application of the maximum effectivevoltage to the heater becomes equal to or longer than a predeterminedheater overheat preventing time; and a voltage decreasing section tocommand the heater regulating section to apply, to the heater, a lowereffective voltage which is lower than the maximum effective voltage andwhich is so set as to hold the temperature of the cell higher than orequal to 500° C. when the application time duration becomes equal to orlonger than the predetermined heater overheat preventing time.
 2. Thegas sensor control apparatus as claimed in claim 1, wherein the gassensor control apparatus further comprises: an activation judgingsection to examine whether the impedance sensed by the impedance sensingsection is smaller than or equal to an activation judging threshold, andjudges that the gas sensor is activated when the impedance is smallerthan or equal to the activation judging threshold; and a time settingsection to set the heater overheat preventing time equal to a firstamount when the impedance is smaller than or equal to the activationjudging threshold, and to set the heater overheat preventing time equalto a second amount longer than the first amount when the impedance isgreater than the activation judging threshold.
 3. The gas sensor controlapparatus as claimed in claim 1, wherein the lower effective voltage isso set as to hold the temperature of the cell higher than or equal to700° C.